The invention relates to sealing elements for dry running piston compressors, in particular for dry running oxygen compressors.
Dry running piston compressors are compressors which operate without foreign lubricants, such as for example lubricating oil, in the compression part. The compression chamber must be sealed off from its surroundings as well as possible in this situation in order to prevent or at least to reduce to a minimum an undesirable or uncontrolled flowing out of the medium to be compressed. Critical points with respect to the sealing are, in particular, the region between the outer wall of the piston and the inner wall of the cylinder in which the piston moves during operation, the region of the passage of the piston rod into the cylinder space and the sealing components of the inlet and outlet valves (suction and compression valves) through which the medium to be compressed is brought into the compression chamber and the compressed medium is ejected from it respectively.
With the term xe2x80x9csealing elementxe2x80x9d all elements are meant which perform a sealing function with respect to the compression chamber, thus in particular guide rings and piston rings for sealing the piston, sealing or restrictor rings for sealing the piston rod and sealing valve components such as valve plates, valve rings or poppets.
In dry running piston compressors in particular sealing elements of this kind are subject to enormously high mechanical and thermal stresses. Since for example the sealing of a piston or of the piston rod is based on dry friction between the counter-running partners, the temperature in the compression chamber can rise sharply depending on the average piston speed and the pressure difference to be sealed off. In addition to good tribological properties, sealing elements of this kind must also retain their sealing properties at high temperatures and at high pressure differences. The sealing valve components must also function without problem in spite of strong alternating stresses and often high temperatures in order to achieve a good efficiency in dry running compressors.
For sealing off the piston and the piston rod in dry running piston compressors in particular, sealing elements are frequently made of plastic, for example of filled polymers or high-temperature polymers. For example polytetraflouorethylene (PTFE), poly(ether ether ketone) (PEEK), polyimide (PI) or polyamidimide (PAI) are used as a polymer material. These polymers are usually mixed with fillers such as carbon, graphite, glass fibres, metals, ceramic or solid lubricants and are designated in this form as filled polymers.
Especially in the compression of oxygen by means of dry running compressors, however, very special problems arise with respect to the materials for the sealing elements as a result of the high reactivity of the oxygen, above all when it is present in a compressed form and high temperatures are present in addition. Here there is the extremely great danger of the arising of compressor fires, which have significantly more devastating consequences in comparison with the compression of other gases. The presence of pure oxygen in compressed form leads to fires which can hardly be extinguished and which thus as a rule lead to the complete destruction of the compressor. In addition there is a considerable risk for the operating personnel.
For these reasons, for oxygen compressors, enormously strict requirements are placed on their operating safety. It has proved that most of the known polymers or high-temperature polymers respectively, which are admittedly used successfully in the compression of other gases, are not suitable for the manufacture of sealing elements for dry running oxygen compressors because they do not satisfy the strict safety requirements. Of the polymers usually used in dry running compressors as a sealing material only PTFE has proved to be xe2x80x9coxygen qualifiedxe2x80x9d up till now. Other high-temperature polymers such as for example PEEK are not suitable and are therefore not used for oxygen compressors for safety reasons.
Sealing elements of PTFE or filled PTFE have other disadvantages, however. Thus PTFE has a pronounced tendency to creep (high cold flow) so that the pressure differences which can be achieved are strongly restricted hereby. High pressure differences lead to a flowing of the sealing elements, for example of the piston ring, through which the sealing action becomes so poor that unjustifiably high leakage rates arise. Therefore dry running oxygen compressors can only be operated nowadays at relatively low pressure differences in comparison with other gas compressors. This is however unsatisfactory in respect to an efficient and economical operation at a high efficiency.
The object of the invention is therefore to provide sealing elements for dry running piston compressors, in particular for dry running oxygen compressors, which also enable an efficient operation of the compressor at high pressure differences. The sealing elements should especially satisfy the high safety requirements for the dry running sealing of oxygen.
The sealing element satisfying this object is characterised by the features of the independent claim 1.
High pressure differences can also be efficiently sealed off with sealing elements which contain perfluoroalkoxy-copolymer (PFA). It is unexpected and surprising in particular that it has proved that such sealing elements in accordance with the invention also satisfy the high safety requirements which are placed on sealing elements for dry running oxygen compressors. The mechanical, thermal, tribological and chemical properties of the sealing element in accordance with the invention enable a significant increase in performance, in particular high pressure differences, in dry running oxygen compressors in comparison with those oxygen compressors which contain known sealing elements, without concessions to the operating safety being required.
The sealing elements in accordance with the invention have the additional advantage that they can be manufactured by means of injection moulding processes.
The sealing elements in accordance with the invention preferably further contain fillers such as carbon powder, graphite, bronze, copper, molybdenum disulphide and/or fibrous materials such as carbon fibres, aramide fibres, glass fibres and/or glass balls. Through such fillers e.g. the tribological properties can improved, the heat resistance can be increased and the cold flow can be practically reduced to zero.
The sealing element in accordance with the invention is preferably designed as a piston ring or a guide ring for the piston, as a sealing or restrictor ring for sealing off the piston rod or as a sealing valve component, in particular as a valve body, of a valve of the piston compressor.